EP0278096A2 - Method for continuously controlling emissions and immissions of mercury - Google Patents

Abstract

In the method, samples of exhaust gases are collected and are quantitatively determined for their Hg content regardless of the form it takes. The object of the invention is to design the above method in such a way that the Hg can be continuously monitored both on the emission and on the immission side. This object is achieved in that a) the partial flows collected from the exhaust gases are passed through a converter system in which the mercury is converted to atomic form, and in that b) the exhaust gases re-emerging from the converter system are fed to a measuring device in which the concentration of the atomic mercury is detected by means of chemical and/or physical methods.

Description

The invention relates to a method for the continuous monitoring of emissions and immissions in relation to mercury, samples being taken from exhaust gases and their Hg content being determined quantitatively irrespective of the manifestation.

When fuels containing mercury are burned, mercury is practically completely released into the exhaust gas due to its high volatility. Mercury is quantitatively oxidized to mercury (II) chloride by the hydrogen chloride gas present and then emitted from the chimney if no suitable exhaust gas purification processes are available. If the exhaust gas is cleaned by wet scrubbers, then in addition to the non-quantitatively separated Hg (II) component, elemental Hg formed by interactions with the exhaust gas can also be emitted from the system.

Only a few of the waste incineration plants in operation can comply with the emission limits for mercury of 200 µg / Nm³ prescribed in TA-Luft. Because of the high toxicity of the various forms of mercury, a MAK value of 100 µg / Nm³ (mercury metal and inorganic mercury compounds) was established for the exposure. In addition, it must be possible to reliably monitor the emission and immission limit values regardless of the form of mercury.

Continuous monitoring of the defined emission and immission limit values is currently not possible with regard to mercury. The limit values are monitored discontinuously according to the following 2 procedures:

1. Absorption in solutions

Regardless of its appearance, mercury can be absorbed quantitatively from the exhaust gas in strongly acidic oxidizing scrubbing solutions. For this purpose, a partial stream of the mercury-containing exhaust gas is passed through two impingers arranged one behind the other, filled with nitric acid peroxodisulfate solution or sulfuric acid permanganate solution. After analyzing the mercury content in the absorption solution, the corresponding mercury emission is calculated taking into account the exhaust gas volume.

2. Adsorption on solids

Various solid adsorbers are available for the detection of elemental and inorganic mercury. Iodine carbon is particularly suitable. For this purpose, a partial stream of the mercury-containing exhaust gas is passed through a heated carbon iodide adsorber and the adsorber is then chemically digested in order to be able to calculate the corresponding mercury emission from the amount of mercury adsorbed with a known exhaust gas volume.

These discontinuous measuring methods can only be carried out with considerable effort. Analytical information cannot be obtained at the sampling location. The amount of mercury contained in the exhaust gas and therefore also in the exhaust gas is subject to strong fluctuations. Discontinuous measurements, such as those occasionally carried out by the responsible monitoring bodies, only lead to statements about emission conditions during sampling. A satisfactory monitoring of the actual emissions situation is not possible. Ge Health risks for persons who are exposed to mercury cannot be excluded with sufficient certainty through discontinuous monitoring.

The object of the invention is that e.g. To design processes in such a way that the Hg can be continuously monitored both on the emissions and on the emissions side.

The solution is described in the characterizing features of claim 1.

The remaining claims indicate advantageous developments of the method.

According to the invention, therefore, a suitable converter system has been developed with which the primarily occurring mercury (II) halides can be continuously converted into the detectable atomic mercury form. By combining this converter with e.g. A common atomic absorption spectrometer then offers the possibility of continuous monitoring in the various areas of application.

The present invention makes experimentally validated proposals for the continuous conversion of the gaseous mercury (II) halides which occur during emissions and immissions into the measurable atomic mercury form. The conversion can take place either by chemical reduction with carbon or by direct thermal decomposition.

The mercury (II) halides present in the exhaust gas or ambient air are tempered by being passed through, for example Carbon, for example in the form of activated carbon, carbon-containing solids or a lime-carbon mixture, is completely reduced to the atomic form.

The optimal conversion conditions are achieved at reaction temperatures of 330 ° C. At this temperature, the ratio of mercury desorption and carbon volatilization is particularly favorable. Large amounts of mercury, such as those present in the raw gas of incineration plants, can be completely converted into the atomic form by carbon with residence times of just 0.04 s. The usual hydrogen chloride concentrations of 250 mg / Nm³ in pure gases have almost no influence on the conversion reaction. For raw gas measurements in which HCl concentrations of more than 1000 mg / Nm³ occur, a suitable absorber must be used for HCl separation. For this purpose, for example, the coal can be mixed with an appropriate amount of lime and this mixture can be used simultaneously as an HCl absorber and an Hg converter. With the carbon converter, the low mercury levels in ambient air can also be reliably converted into the atomic form.

The invention is described in the following using two exemplary embodiments for the conversion reaction of Hg. 1 and 2 and with the aid of experiment examples 1 to 10 with results correspondingly shown in FIGS. 3 to 9.

A continuous conversion of mercury (II) compounds into the atomic mercury form can be carried out with a converter unit according to FIG. 1. The converter unit 1 consists of a heating jacket 2 and a reaction tube 4 filled with carbon 3. The heating jacket 2 is electrically heated and has a hinged lid, so that the reaction tube 4 can be conveniently removed or inserted. The Heating coils of the heating jacket 2 can be inserted in ceramic multi-hole bodies, the channels of which are open to the usable space. To protect against possible contact with the heating conductor, the power consumption is automatically interrupted when the heating jacket 2 is opened and switched on again when it is closed. The temperature is monitored by a sensor 5 in the heating jacket 2 and kept at the predetermined value via a control system. The reaction temperature is continuously adjustable in the range from 300 to 400 ° C. The temperature profile of the heating jacket is constant in the area of the coal 3.

The reaction tube 4 can be made of Duran glass. In order to achieve the lowest possible flow resistance in the reaction tube 4, the carbon 3 used for the reduction should be in coarse-grained form (diameter 1 to 3 mm). The carbon 3 is fixed by two glass wool plugs 6 and 7 in the glass tube so that it is in the temperature-constant zone of the heating jacket 2. The distance from the converter unit 1 to the detection system (not shown) should be kept as short as possible in order to rule out condensation effects.

The thermal decomposition can take place in a converter unit 8 according to FIG. 2, consisting of an electrically heated jacket 9 and a reaction tube 10. The heating jacket 9 can consist of a ceramic carrier tube, on which the heating conductor is placed as an outer winding and is embedded with ceramic material. The temperature is monitored by a sensor 11 in the heating jacket 9 and set to the predetermined value using a control unit. The working temperature can be continuously adjusted in the range 700 to 900 ° C. The thermal decomposition can take place in the reaction tube 10 made of quartz glass. In order to achieve the required dwell times with the smallest possible dimensions, the reaction tube 10 is arranged in a spiral and preferably opens in the middle of the converter unit in the absorption cell 12 of a spectrophotometer.

In principle, the converter unit 8 can be designed such that both carbon reduction and thermal decomposition can be carried out in the same heating jacket. The converter system consists here, in a manner similar to that described in the first example according to FIG. 1, of a hinged heating jacket. The working temperature is continuously adjustable in the range 300 to 900 ° C. The dimensions of the heating jacket must be designed so that the required dwell times are achieved for both types of conversion.

The conversion reaction in relation to mercury (II) chloride was considered as representative of all mercury (II) halides. Carbon in the form of granular activated carbon AK (⌀ 1-3 mm) was used for the reduction. The forms of mercury were identified using the Dowex (Hg (II)) - iodine (Hg ⁰ ) adsorber combination.

Example 1:

Each 10.0 g of activated carbon was heated for 24 hours at different temperatures and then the percentage loss in mass was determined by weighing. The test results are shown in Fig. 3.

Example 2:

A clean gas stream with a mercury content of 100 µg / Nm³ was passed through a glass tube filled with activated carbon at different temperatures. The composition of the clean gas flow corresponded to that of an incineration plant (SO₂ = 200 mg / Nm³, NO _x = 200 mg / Nm³, CO = 100 mg / Nm³, HCl = 100 mg / Nm³, O = 11%).
The test results are shown in Fig. 3.

Example 3:

At a temperature of 350 ° C, a clean gas stream with 1000 µg Hg / Nm³ with different residence times was passed through activated carbon.
The test results are shown in Fig. 4.

Example 4:

At a temperature of 350 ° C, a clean gas stream with 100 µg Hg / Nm³ at different HCl concentrations was passed through activated carbon.
The test results are shown in FIG. 5.

Example 5:

At a temperature of 350 ° C, a clean gas stream with 100 µg Hg / Nm³ at different HCl concentrations was passed through a mixture of activated carbon and calcium oxide.
The test results are shown in FIG. 5.

Example 6:

Die Experimentbeispiele 7 bis 10 sind mit der thermischen Zersetzung mittels der Konverteranlage gemäß Fig. 2 ausge­führt worden.At a temperature of 350 ° C, ambient air with different mercury levels was passed through activated carbon.

Experiment examples 7 to 10 were carried out with the thermal decomposition by means of the converter system according to FIG. 2.

The mercury (II) halides present in exhaust gas or ambient air are completely decomposed as they pass through the sufficiently heated temperature zone to form atomic mercury.

The thermal decomposition of the mercury (II) halides is already noticeable at temperatures around 600 ° C. Complete decomposition takes place at reaction temperatures of 800 ° C. (Example 7) with gas residence times of 0.4 s (Example 8). The residence times required for complete conversion decrease with increasing temperature. The presence of hydrogen chloride gas counteracts the decomposition reaction. Even in the presence of 100 mg HCl / Nm³, the formation balance is largely shifted to the side of Hg (II) again (Example 9).

Thermal decomposition can only be used without hesitation if the presence of hydrogen chloride can be largely ruled out. This is usually the case with emissions monitoring. If, on the other hand, the presence of HCl must be assumed, this component must be removed. This is possible, for example, by adding a temperature-controlled HCl absorber, for example calcium oxide or silica gel (example 9). However, the absorber efficiency e.g. with bromophenol blue, a reagent for HCl. Direct thermal decomposition in the absorption cell with simultaneous atomic spectrometric detection is possible while maintaining sufficient residence times and excluding hydrogen chloride (Example 10).

Example 7:

A clean gas stream (without HCl) with 1000 µg Hg / Nm³ was passed through a differently heated temperature zone.
The test results are shown in FIG. 6.

Example 8:

A clean gas stream (without HCl) with 1000 µg Hg / Nm³ was passed through a temperature zone heated to 800 ° C with different dwell times.
The test results are shown in FIG. 7.

Example 9:

A clean gas stream with 1000 µg Hg / Nm³ was passed at different HCl contents through a temperature zone heated to 800 ° C without or with an upstream CaO absorber.
The test results are shown in Fig. 8.

Example 10:

A clean gas stream (without HCl) with 1000 µg Hg / Nm³ was passed through a differently heated absorption cell of a spectrophotometer (AAS) with a residence time of 0.1 s.
The test results are shown in FIG. 9.

Claims (6)

1. A method for the continuous monitoring of emissions and immissions in relation to mercury, wherein samples are taken from exhaust gases and their Hg content is determined quantitatively regardless of the manifestation, characterized in that a) the partial streams taken from the exhaust gases are passed through a converter system (1 or 8) in which the mercury is brought into atomic form, and that b) the exhaust gases emerging again from the converter system (1 or 8) are fed to a measuring device in which the concentration of the atomic mercury is detected by means of chemical and / or physical methods. 2. The method according to claim 1, characterized in that the exhaust gases are passed through a solid and / or a temperature field (9, 10). 3. The method according to claim 1 and 2, characterized by the use of atomic absorption spectrometry as evidence of the atomic mercury. 4. The method according to claim 1 or one of the following, characterized in that as a solid bed (3) coal, in particular activated carbon, is used and the solid bed (3) is kept at temperatures around 350 ° C. 5. The method according to claim 1 or one of the following, except claim 4, characterized in that the sample gas is passed through a heating zone (9, 10) at temperatures above 700 ° C. 6. The method according to claim 1 or one of the following, characterized in that a Ca-containing substance is added to the sample gas stream (10) or the solid bed (3).

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